1. Field of the Invention
The present invention generally relates to a cathode for use in electron tubes such as, for example, picture tubes or camera tubes and, more particularly, to an improvement in electron emissive material deposited on a surface of the cathode. The present invention also relates to a method of making the cathode of the type referred to above.
2. Description of the Prior Art
It is well known that the cathode in a cathode ray tube plays an important role of emitting electron beams when heated. An example of the prior art cathode is illustrated in FIG. 5 of the accompanying drawings in longitudinal sectional representation, reference to which will not be made for the discussion of the prior art.
The illustrated cathode is made of a base metal 1 containing as a principal component nickel mixed with a slight amount of reducing metal such as magnesium and/or silicon and is comprised of an open-ended tubular cathode body 1a and a cathode cap 1b mounted under interference fit on one open end of the tubular cathode body 1a so as to close the opening at such one open end. The cathode also comprises a heating element 2 built in the tubular cathode body 1a and an electron emissive layer 3 of electron emissive material deposited on an outer surface of the cathode cap 1b. The electron emissive material forming the electron emissive layer 3 is generally prepared by mixing a predetermined percent by weight of barium carbonate (BaCO.sub.3) and a perdetermined percent by weight of scandium oxide (SC.sub.2 O.sub.3) into a resinous solution, which is prepared by dissolving nitrocellulose with the use of an organic solvent, to provide a suspension, and then applying the suspension to the outer surface of the cathode cap 1b to form the electron emissive layer 3 by the use of a spray technique, an electro-deposition technique or a painting technique after the particle size of the solid components in the suspension has been adjusted.
As hereinabove discussed, in the prior art electron tubes, a so-called `oxide cathode` is largely employed in which a layer of oxide of an alkaline earth metal containing barium (Ba) is deposited on the outer surface of the cathode cap 1b. The oxide cathode is operable as an electron emissive donor which emits electron beams when, after a carbonate of the alkaline earth metal has been transformed into an oxide upon pyrolysis, the reducing metal and the oxide react with each other to cause the oxide to form free atoms. The reason that the oxide cathode undergoes such a complicated process to emit the electron beams is because it employs as a starting material the carbonate which is chemically stable. More specifically, since the barium (Ba) is a highly active material although it has a relatively high power of emitting electrons, it tends to produce barium hydroxide (Ba(OH).sub.2) upon reaction with a water component contained in the air and, therefore, once the barium hydroxide is formed, it is difficult to cause the barium hydroxide to produce free barium (Ba) within the envelope of the electron tube.
The carbonates are available in the form of a single element such as barium carbonate (BaCO.sub.3) and also in the form of a multi-element such as carbonates of alkaline earth metal (Ba, Sr, Ca)CO.sub.3, and all of these compositions are identical so far as the fundamental mechanism of activation is concerned.
The cathode of the above described construction is incorporated in the envelope of the electron tube, which envelope is subsequently highly evacuated during an evacuating step. During the evacuation, the heating element 2 is activated to heat the interior of the envelope to a high temperature of about 1,000.degree. C. When the envelope is so heated, the barium carbonate (BaCO3 ) undergoes the following pyrolysis. EQU BaCO.sub.3 .fwdarw.BaO+CO.sub.2 ( 1)
Carbon dioxide formed as a result of the reaction (1) above is discharged to the outside of the envelope. At the same time, resins such as nitrocellulose are also thermally decomposed into a gaseous body which is also discharged to the outside of the envelope together with the carbon dioxide. The reaction (1) brings about transformation of the barium carbonate (BaCO.sub.3) of the electron emissive layer 3 into barium oxide (BaO). According to the prior art cathode, during the reaction (1) above taking place, reducing metals such as silicon (Si) and magnesium (Mg) which play an important role in reducing reaction are oxidized together with nickel (Ni) on the surface of the cathode under the oxidizing atmosphere within the envelope which contains carbon dioxide (CO.sub.2) and oxygen (O.sub.2).
FIG. 6 illustrates, on an enlarged scale, the joint between the base metal 1 and the electron emissive layer 3. In general, the barium oxide (BaO) transformed from the barium carbonate (BaCO.sub.3) is in the form of an aggregation 9 of generally rod-shaped crystalline particles 8 of some micrometers to some tens micrometers in particle size, having fine interstices 10 defined among the crystalline particles 9 to form the electron emissive layer 3 which is porous in structure. At the interface between the electron emissive layer 3 and the base metal 1, the barium oxide (BaO) reacts with the reducing metals such as silicon (Si) and magnesium (Mg) to form free barium (Ba). These reducing metals are diffused into interstices 7 defined among crystalline particles 6 of nickel (Ni) forming the base metal 1 and undergoes a reducing reaction in the vicinity of the interface 11 between the base metal 1 and the electron emissive layer 3.
Examples of the reaction taking place at the interface 11 are illustrated below. EQU 2BaO+Si.fwdarw.2Ba+SiO.sub.2 ( 2) EQU BaO+Mg.fwdarw.Ba+MgO (3)
The free barium (Ba) formed as a result of the reaction of the formula (2) participates in the electron emission as an electron emissive donor. At the same time, the following reaction takes place. EQU SiO.sub.2 +2BaO.fwdarw.Ba.sub.2 SiO.sub.4 ( 4)
Although the electron emissive donor referred to hereinbefore is formed at the joint between the electron emissive layer 3 and the base metal 1 and moves through the interstices 10 in the electron emissive layer 3 shown in FIG. 6 to the outer surface of the electron emissive layer 3 for the participation in electron emission, the electron emissive donor is susceptible to evaporation and also to loss as a result of reaction with gaseous bodies of CO, CO.sub.2, O.sub.2 and H.sub.2 O remaining within the envelope. Therefore, the electron emissive donor must be replenished by the above described reactions and, therefore, the reducing reaction takes place at all times during the operation of the cathode. In order to make a balance between the replenishment and the loss, the prior art cathode is required to be operated at about 800.degree. C.
Also, as the reaction formulas (2) and (4) make it clear, during the operation of the cathode reaction products 12 such as SiO.sub.2, Ba.sub.2 SiO.sub.4 and others are formed at the interface 11 between the electron emissive layer 2 and the base metal 1 and are then accumulated in the interface 11 and the interstices 7 to form a barrier (hereinafter referred to as an intermediate layer) for the passage of silicon (Si). The presence of the barrier, that is, the intermediate layer, tends to delay the reaction making it difficult to form barium (Ba) which is the electron emissive donor.
In order to eliminate the above discussed problems, in any one of numerous patent literatures, for example, U.S. Pat. No. 4,518,890, issued May 21, 1985; U.S. Pat. No. 4,007,393, issued Feb. 8, 1977; U.S. patent application Ser. No. 864,566, filed May 16, 1986 now U.S. Pat. No. 4,864,187 (corresponding to a combination of Japanese Laid-open Patent Publications No. 61-269828 and No. 61-271732, published Nov. 29, 1986, and Dec. 2, 1986, respectively); U.S. patent application Ser. No. 886,777, filed Jul. 17, 1986 now U.S. Pat. No. 4,797,593 (corresponding to a combination of Japanese Laid-open Patent Publications No. 62-22347, No. 62- 165832, No. 62-165833, No. 62-90821, No. 62-198029, No. 62-193032, No. 62-90820, No. 62-193031 and No. 62-88239, published Jan. 30, Jul. 22, Jul. 22, Apr. 25, Sep. 1, Aug. 24, Apr. 25, Aug. 24, and Apr. 22, 1987, respectively); and U.S. patent application Ser. No. 204,818, filed Jun. 10, 1988 now U.S. Pat. No. 4,980,603 (corresponding to Japanese Laid-open Patent Publications (No. 63-310535 and No. 63-310536, both published Dec. 19, 1988), there is disclosed the use of scandium oxide (Sc.sub.2 O.sub.3) dispersed in the electron emissive layer 3 so that the reaction products 12 such as Ba.sub.2 SiO.sub.4 shown in FIG. 6 can be dissociated in the presence of scandium (Sc). However, it has been found that, since globular crystalline particles of the scandium oxide employed therein does not sufficiently disperse into the electron emissive layer 3, it often occurs that the dispersion of scandium oxide (Sc.sub.2 O.sub.3) will bring about little effect as compared with the cathode in which no scandium oxide is dispersed, and therefore, the cathode in which the scandium oxide is dispersed will not give a stabilized effect.
As hereinbefore discussed, in the prior art cathode for use in the electron tubes, not only do both of the oxidization of the reducing metal and the accumulation of the reaction products occur during the reaction to decompose and reduce carbonates for the formation of the electron emissive donor, but also during the operation of the cathode the reaction products 12 tend to be accumulated in portions of the nickel crystalline interstices 7 in the vicinity of the base metal 1 and the electron emissive layer 3, particularly in the vicinity of the outer surface of the base metal 1 adjacent the electron emissive layer 3. Therefore, the dispersion of the reducing metal into the electron emissive layer 3 tends to be progressively disturbed to such an extent that no satisfactory electron emissive characteristic can be exhibited under a high electric current density for a prolonged time. In addition, since the resultant electron emissive layer 3 in the prior art cathode is not sufficiently porous in structure, the electron emission is not satisfactory.